A hydraulic excavator is currently widely used in occasions of building constructions such as housing foundation excavation and backfilling, pipeline laying and farmland water resource construction. An excavator has advantages in allowing flexible construction and having high execution efficiency. A hydraulic excavator is mainly configured by components such as a work device, a rotation mechanism, a travel mechanism, a rotating platform, and a hydraulic control device. Lifting and lowering of a boom, extending and contracting of a stick, rotating of a bucket and rotating of a rotary device are all realized by the control device to control a multiplexer valve group to allocate hydraulic oil supplied to hydraulic pumps via a hydraulic circuit. An excavating operation at a construction site usually involves a combination of a long-time excavating operation and a land grading operation. Therefore, under the premise of not affecting operation efficiency, both excavating operation characteristic and grading operation characteristic of an excavator are highly demanded by an operator.
The so-called excavating operation characteristic refers to a comprehensive judgment regarding stability, smoothness, and maneuver responsiveness of an excavator during an excavating operation. Among these, the smoothness and the maneuver responsiveness of the excavating operation are of most concern to an operator. A reasonable ratio of movement speeds of a boom cylinder and a stick cylinder of an excavator directly determines the excavating operation characteristic of the excavator and directly affects use efficiency of the excavator and operation experience of the operator. Best matching of cylinder movement speeds is achieved by controlling the openings/closings and a degree of opening of a valve spool of a hydraulic control valve via pilot oil of a hydraulic control system of the excavator. A valve corresponding to the valve spool is opened to connect to an oil passage of a main pump. Controlling over extension and contraction and a speed of a cylinder movement are achieved by connecting oil passages and by controlling flow rates of the oil passages.
During an excavating operation of a conventional excavator, a work device has a large acceleration. A sudden stop at a high speed or a sudden start is often accompanied with large shaking and rocking of the whole excavator, which affects comfortability and smoothness of an excavating operation of an excavator and increases tiredness of an operator and thus affects operation efficiency. Lifetime of an excavator operating under such a condition is reduced. Further, frequent switching of a hydraulic pump between no-load and high-load states increases engine load. Fuel consumption and operating costs both increase.
Grading operation characteristic of an excavator is an important use characteristic after the excavating operation characteristic, and is mainly used in the following work conditions: excavating a trench for pipe laying; land grading, slope repairing, and the like. In a conventional technology, the grading operation characteristic of an excavator can be directly linked to design parameters of the excavator. Once a product design is completed, movement speeds of a boom cylinder, a stick cylinder and a bucket cylinder are fixed. Therefore, during a land grading operation, the movement speed of the boom, stick and bucket is a constant parameter. A conventional excavator has the following problems during a land grading operation: Boom lifting is too fast; reciprocating speed of a stick is poor; and the bucket speed is not coordinated and synchronized with a combined movement of the boom and the stick. Thus, performance of a grading operation is directly affected.
During a grading operation, a main reason for slow lifting of a boom is that, in a conventional design, an oil supply method of using double pump confluence is adopted for increase excavating speed of a bucket. In double pump oil supply, oil is simultaneously supplied to a boom cylinder, a stick cylinder and a bucket cylinder. When an excavator performs a lifting operation during excavating or trenching, since a large amount of hydraulic oil is lost too early due to confluence of the bucket cylinder, amount of hydraulic oil supplied to the boom cylinder is reduced, causing uncoordinated moving speeds. In fact, directly adding a corresponding flow control valve in a circuit of the boom cylinder, the stick cylinder and the bucket cylinder can also adjust to some extent a combined operation effect of an executing element to alleviate a speed mismatch problem. However, because a size of a flow-control orifice is hard to be reasonably determined, effects obtained by adopting this method do not provide generalized significance. In addition, this method increases complexities and difficulties in pipeline design and adjustment, and occupies the limited control pipeline of a control valve.